17" vs. 18"

[Allen K. Littlefield]

I just measured my '02s rubber from the rim to the road and got 2 3/4" of sidewall. The tires are Continenal Sport Contact 2, 205/50ZR 17" Can any of you tell me the amount of side wall on a Carrera 18" rim? Front wheel measured. I am considering the 18" spoke Carrera type rims but am worried about potholes. If the rubber to the road ratio is about the same, then I have the problem on my 17" that I am trying to avoid. Porsche now has cars new with 19" rims so I don't think it should be too big a problem (until you bury one in a 6" deep pothole, goes for 17 and 18s too I guess). Also is the width of the Carrera spoke wheels different than similar spoke wheels put on at the dealership? I see a lot of 10" wide 18" wheels out there but have heard the Boxster "version" is only 9" wide. Also is the side walls on the 18s stiffer than the 17s? Any thoughts?

AKL

[Sammy]

You can do the formula but the overall outside tire diameter is suppose to remain the same within a certain percetage so theoretically by going from a 17" to an 18" you will lose 1" of total sidewall. This translates to 1/2" each side so 2 3/4 - 1/2 = 2 1/4" worth of sidewall.

If you want the more precise formula go to Tire Rack's site and it will provide a detailed explanation on how to use the tire's size marking to translate into width and sidewall.

I don't think the sidewall is any stiffer on an 18" than a 17" tire of the same brand and model. You might feel an increase in stiffness but it would be because you are taking the 1" diameter difference and changing from rubber to metal.

Sorry I can't help with the wheel questions.

[racer_d]

The 986 Boxster rear maximum width from the factory is 9"
The 996 Carerra (narrow body/ non Turbo body) rear maximum width from the factory is 10"

Going from 17" to 18" is a "plus 1" jump up. you trade rim diameter for sidewal thickness. The OVERALL diameter of the tire/rim combo (the wheel) will be within likely 2% difference.

18"s will provide crisper handling response (smaller sidewall) and worse pothole/bump protection and smoothness (smaller sidewall).

[hamstur]

Here's how to read your tire...

205/50/17 means 205mm width, 50% sidewall aspect ratio, and 17" rim size, so 50% of 205mm = 102mm / 25.4 mm per inch = 4 inch sidewalls.

Your tire will compress about 30% when mounted, so 70% * 4 inchs = 2.8 inches, which means your 2.75 inch measurement is correct.

The OEM front tire for a 996 is 225/40/18, so their height is: 40% of 225 mm = 90mm / 25.4 mm per inch = 3.5 inches * 70% = 2.45 inches.

Porsche does not recommend 19" on 986s and 996s because of the car design...the harsh ride and vibration caused windshields to crack. So...the answer to your question is yes, the sidewalls on the 18" tires will feel stiffer because there is 0.5 inch less sidewall.

If the tires are a bit wide, you would have to shell out a couple hundred for spacers.

Hope this helps,
Andy

[Perfectlap]

I fear 10" rears because they require spacers, and those are not ideal for raced/hard driven cars. The fit has to be PERFECT and even with a perfect fit there is more pressure on the wheel bearings which are ridonkulusly expensive for this car.

[Allen K. Littlefield]

Thank you all for responding. I never liked the idea of spacers as I remember the wide bodied 911 guys that used spacers and they were never satisfied. I like the ride and feel of the 17" rims that I now have and the ride is firm but not harsh unless the road is really bad but that cannot be helped. I do love the look of the 18" and Porsche did offer them as a option (Porsche is now offering 19" on some cars). I notice a lot of guys on this forum have switched to 18" and some are "cheating death" on 19", I wonder if they are experiencing problems?

AKL

[Thumper]

When I changed out my 17" stock wheels for 18" Gemballa Competitions, I honestly could not tell any difference in ride quality or handling. But then again, I don't track my car......yet. The front's are 8.5" wide and the rears are 10" wide. No spacers were required and they fill out the fenderwells perfectly. Spacers may be required with Porsche wheels, but an aftermarket wheel can be bought with the correct backspacing so spacers aren't required.

[Ronzi]

Originally Posted by Allen K. Littlefield

I just measured my '02s rubber from the rim to the road and got 2 3/4" of sidewall. The tires are Continenal Sport Contact 2, 205/50ZR 17" Can any of you tell me the amount of side wall on a Carrera 18" rim? Front wheel measured.
AKL

My '99 has 8x18 and 10x18 Sport Design style wheels.
Measuring the fronts I get 2 3/4" from garage floor to front wheel rim flange, Conti SportContact, 225/40R18, inflated to 34psi, 60degF ambient.
My Pirelli PZero 17s are in a stack next to the car, so I can't measure them loaded, but unloaded they measure 3" from flange to tread surface. Loaded I expect I would get pretty much the same dimension you did, 2 3/4".
All of which would seem to indicate you won't see a whole lot of real-world difference in sidewall height between the 17s and the 18s. Not to say there won't be some, but it won't be much.

[Ronzi]

I must be suffering from a severe case of brain fade or something this morning, because after I posted the above I got to thinking, did I actually take the measurement at the absolute bottom of the rim?
After rechecking, I found that I had not. The measurement on the 18" wheel should be 2 1/2", yielding a more believable 1/4" difference.
Sorry for the error.
Hamstur was right on the button.

[Allen K. Littlefield]

Appreciate the responses and insight. For now I will run the 17" all this summer and perhaps make a change this fall or next spring. Certainly won't be 19" wheels for sure.

AKL

[funster]

My car came with 18's and other than looking cool, they don't seem to serve any purpose. A 17 would absorb more road shock and the rim would be less likely to get bent. Also on a 17, less weight is on the outer circumference of the wheel which should translate to better acceleration, however slight it may be. Some say that the shorter sidewall will give better response, but I believe there is a point of diminishing returns on this. You don't see race cars running ultra low profiles.